Total Hardness is a measure of the calcium and magnesium concentration in water, though usually we are only concerned with calcium. “Hard” water has a high level of calcium, typically above 200 ppm, or 12 grains.
Though you might think soft water (below 50 ppm) is good, it is not for a spa. It turns out that water without minerals dissolved in it, commonly called distilled water, is slightly corrosive. Over time, soft water can pit the surface of metal parts in your spa.
Calcium in your spa water also helps reduce foaming. Hard water is called “hard” because its hard to develop soap suds with hard water.
Lastly, though not as much as Total Alkalinity, calcium also serves as a bit of a pH buffer. It helps stabilize the pH in your spa water.
